Coral morphology and evolution

The class Anthozoa includes soft-bodied sea anemones and corals that build calcareous skeletons. This ability to biominer-alize arose at least four times within the group, once for each of the major skeletal orders, and once for an unusual Silurian coral called Kilbuchophyllia (Fig. 4.2). Coral skeletons can be made from aragonite or calcite (both CaCO3). Most tabulate and rugose corals are built from the latter, most scleractinian skeletons from the former.

Tabulate and rugose corals evolved from soft-bodied anemone ancestors in the Ordovician period. They thrived in the Silurian, Devonian, and, following a late Devonian decline, in the Carboniferous. They were part of the Palaeozoic reef community, but lacked a holdfast and so did not form the framework of reefs as modern corals do. They became extinct in the end-Permian mass extinction. Scleractinian u o

lij CC cd ca

u corals evolved in the Triassic and radiated throughout the Mesozoic. Though many genera became extinct during the end-Cretaceous mass extinction event, they have come to dominate Cenozoic and modern reefs.

Many modern corals have a symbiotic relationship with an alga that lives within their tissue. These organisms, known as zooxanthellae, are protected by the coral and in return are "farmed" for nutrients. They change the internal chemistry of the animal, making aragonite secretion easier. These hermatypic corals are typically colonial. They must live within the photic zone so that their zooxanthellae can photosynthesize. They require clear water less than 30 m deep, and thrive in sea temperatures of between 23 and 29°C. Ahermatypic corals live without an algal symbiont. They are typically solitary corals inhabiting deeper water. Some rugose and tabulate corals may have had zooxanthellae, but no direct tests for this hypothesis are available.

Corals grow a calcareous cup (corallite) surrounding the lower part of their soft tissues (polyp) (Fig. 4.3). As the coral grows, or buds, more mineral is secreted and the structure grows. The whole growth history of a coral is preserved in its skeleton. Within the corallite, a variety of vertical or horizontal structures are built to support the polyp. The most important vertical structures are radial septa (singular septum). The most important horizontal structures are plate-like tabulae (singular tabula) and smaller, upwardly curved dissepiments. Colonial corals have varying degrees of contact between the soft tissues of adjacent polyps. Sometimes the boundary walls between polyps are perforated, sometimes they are lost completely. The whole coral colony is called the corallum, and it can form a range of shapes, from erect, branching forms to low domes.

Fig. 4.3 Section through a generalized coral polyp to show the soft-part morphology. Medusoid stages are similar in morphology, but live "upside down" relative to this diagram, with the mouth hanging below a soft bell of tissue, and no skeleton.

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